Изучение реакций слияния ядер на пучках 4,6He и 7Li

• Автор: Абдель Гхани Абдель Фаттах Хассан Эль-Сайед
• Шифр специальности: 01.04.16
• Научная степень: Кандидатская
• Год защиты: 2004
• Место защиты: Дубна
• Количество страниц: 86 с. : ил.

Contents
Introduction
1 Experimental and Theoretical Situations
1-1 Experimental situations for study of fusion reactions with exotic nuclei:
1-1-1 Cross section for fusion-fission reaction
1-1-2 Cross section for fusion-evaporation reaction
1-1-3 Conclusion
1-2 Theoretical predictions for study of fusion reactions with exotic nuclei:
1-3 Conclusions
2 Experimental Procedures
2-1 Beam production
2-2 Beam monitoring and energy measurement
2-3 Setup for on-line measurements of fission and evaporation cross sections
2-4 Experimental procedure for off-Line measurements of evaporation channel cross sections
3 Experimental Results
3-1 Fission and evaporation cross section measurements in the reactions 4He+209Bi and 6He+209Bi
3-2 Fission and evaporation cross section measurements in the reactions 7Li+208Pb and 7Li+209Bi
4 Experimental Results Analysis
Conclusions
References
The study of fusion reactions near and above the Coulomb barrier involving weakly-bound or radioactive beams are one of the most challenging experimental and theoretical problems in nuclear physics. It is well established that the coupling of collective degrees of freedom to the fusion channel enhances significantly the tunneling probability at sub-barrier energies [1]. On the other hand, the low binding energy of radioactive nuclei may cause important loss of incoming flux due to the breakup process.
Several experimental and theoretical studies concerning the fusion of two nuclei under and near the Coulomb barrier were performed in the past [2]. Most of the results were interpreted adequately well within the framework of coupled-channel (CC) calculations and by using either 4 microscopic potentials or phenomenological ones [3, 4]. With the advent of
radioactive beam facilities, the interest in such studies with halo nuclei was renewed due to their specific features, like extended neutron densities, low-lying continuum, and also very low-energy thresholds for breakup. Fusion, like other reaction processes, should be appreciably affected by such features. The experimental results obtained for fusion reactions at energies close to the Coulomb barrier with light unstable beams were reviewed by C. Signorini [5]. The data point out a strong influence due to breakup processes.
From a theoretical point of view, it is expected that fusion cross sections for halo nuclei will present an increase due to the decrease of the potential barrier and the coupling to soft vibrational modes [6]. This increase, however, according to several elaborate but contradictory theories, may be hindered or enhanced due to breakup effects.
%! In different scientific centers, intensive experimental studies are
performed using secondary beams formed from the radioactive products of nuclear reactions. Lately there has been a growing trend to use secondary beams as a means of investigating the interaction cross sections of these exotic nuclei with the target nuclei. These data help getting information on the structure of nuclei far from the line of stability, on the distribution of nuclear matter and on charge radii. However, for unequivocal conclusions there is a question on comparison of these reactions with the reactions caused by particles of a stable beam nucleus.
Several measurements with halo nuclei were visualized through the systems nBe+209Bi [7], 6He+209Bi [8-10], and 6He+238U [11]. The data are presented in Fig. I together with the data of the associated stable isotopes Be+209Bi [7], 4He+209Bi [12] and 4He+238U [11].
The fission with 6He projectiles has been investigated at Dubna [9, 10] • by studying the fission reaction induced by 6He on 209Bi at Ecm>1.5VB

Decay Time (h)
Fig. 3-9: The decay curves of the alpha activity of the 4 rt evaporation residues 21'At in the reaction (6He+209Bi—► 215'4”At).
Decay time (h)
Fig. 3-10: The decay curves of the alpha activity of the 4« evaporation residues 21‘At in the reaction (7Li+208Pb—>2I5'4"At).
III-gamma rays energy spectrum in the reactions 7Li+20SPb:
The y-rays energy spectra were measured in 7Li+208Pb reactions and are shown in Figs. [(3-11)-(3-12)]. The evaporation channels ■210-206)At with different half lives decay by electron-capture with different branching ratio to <210-205,Po, during this decay mode gamma rays with different energies and different intensities are emitted. Once the energies of the y rays from